Pressure fluctuations in NPT-aimd-simulations

[Akshay Malik]

Dear all,

I am doing AIMD simulations using a cuboidal box with an initial edge length of 14.31 angstroms. After running 50 picoseconds in NPT ensemble (at thermostat 333 K temp. and barostat 1 bar pressure) the average pressure comes out to be around 1.2 bar but the instantaneous fluctuations in pressure at each time step are in the order of -10000 to 10000 bar. Please find the attached file having the plot of pressure at each time step. I have the following 3 questions- 

1. I want to know up to which order the instantaneous fluctuations in pressure are ok?

2. What can be done to decrease these instantaneous fluctuations at each time step?

3. Why there are 2 separate temperatures in the out file like one is thermostat temperature other is barostat temperature while the temperature part is only mentioned once in the input file?

TEMPERATURE [K]              =                     310.171              333.009

BAROSTAT TEMP[K]             =          0.652301869614E+03   0.334642361097E+03

Following is the cp2k code section of the input file:

______________________________________________

&MOTION
  &MD
   ENSEMBLE NPT_I
   TIMESTEP [fs] 0.5
   TEMPERATURE [K] 333
   STEPS 100000
   &BAROSTAT
      TIMECON [wavenumber_t] 1000.0
      PRESSURE  1
   &END BAROSTAT
   &THERMOSTAT
    TYPE NOSE
    REGION MASSIVE
    &NOSE
     LENGTH 3
     YOSHIDA 3
     TIMECON [wavenumber_t] 1000.0
     MTS 2
    &END NOSE
   &END THERMOSTAT

______________________________________________

Following is the outfile : *******************************************************************************
 ENSEMBLE TYPE                =                                            NPT_I
 STEP NUMBER                  =                                           107644
 TIME [fs]                    =                                     53822.000000
 CONSERVED QUANTITY [hartree] =                              -0.135935656268E+04

                                              INSTANTANEOUS             AVERAGES
 CPU TIME [s]                 =                       45.30                14.39
 ENERGY DRIFT PER ATOM [K]    =         -0.331821283044E+02  -0.299197457243E+02
 POTENTIAL ENERGY[hartree]    =         -0.136035799755E+04  -0.136034247538E+04
 KINETIC ENERGY [hartree]     =          0.452327402751E+00   0.485632690237E+00
 TEMPERATURE [K]              =                     310.171              333.009
 PRESSURE [bar]               =          0.256385237325E+04   0.129828680436E+01
 BAROSTAT TEMP[K]             =          0.652301869614E+03   0.334642361097E+03
 VOLUME[bohr^3]               =          0.188345152694E+05   0.189171690534E+05
 CELL LNTHS[bohr]             =    0.2660632E+02   0.2660632E+02   0.2660632E+02
 AVE. CELL LNTHS[bohr]        =    0.2664482E+02   0.2664482E+02   0.2664482E+02
 *******************************************************************************